Project Summary. We have shown that regulatory T cells (Tregs) defeat anti-cancer immunity and that their depletion can be therapeutic, but their rapid regeneration is problematic. This project uses our discoveries that B7-H1 immune co-signaling facilitates Treg generation and that estrogen receptor (ER)? signals inhibit Treg regeneration in relevant pre-clinical mouse models with proven substantial translational relevance. Initial studies focus on ovarian cancer (OC) and use melanoma models to demonstrate concepts in additional tumors, as our discoveries should be applicable to a wide variety of cancers. Effects of B7-H1 on ER? signaling and relations to immune pathology and clinical outcomes are studied. Our overarching objective is to identify novel and effective immune therapy for cancers, with a focus on OC. Our overarching hypothesis is that B7-H1 blockade will augment Treg depletion as cancer immunotherapy and that ER? signals will boost B7-H1 blockade effects. This hypothesis predicts novel approaches to immunotherapy for OC, greatly extends our understanding of its immunopathogenesis and allows development of a similar strategy for other cancers and in men. ER? agonists can be used in males as we have shown and avoid estrogen side effects. The specific aims are Aim 1 Test the hypothesis that ER? signals augment B7-H1 blockade effects in cancer and AIM 2 Test the hypothesis that dysfunctional B7-H1 signaling in cancer is dendritic cell-dependent. These aims are achieved using mice genetically null for signaling components, pharmacologic agents affecting key signaling pathways, and bone marrow chimeras to study hematopoietic versus non-hematopoietic B7-H1 signals. Relevance. We seek to improve treatment options for OC, which kills over 15,000 American women annually, and for which there is no curative option after first-line therapy fails, as it doe in most cases. Principles can be applied to a wide variety of cancers, including melanoma, studied here as a confirmatory second cancer. Our insights into tumor-associated immune dysfunction promise to help improve the efficacy of cancer immunotherapy, whose record of success to date has been only modest.